1. Field of the Invention
The present invention relates generally to a standing-wave vibration motor which obtains a driving force by virtue of standing waves. More particularly, the invention relates to a drive circuit of the above type of vibration motor.
2. Related Background Art
A traveling-wave vibration motor which is becoming a focus of attention has the well-known advantages of facilitating low-speed driving and obtaining a high torque, and is already commercially available in the field of optical equipment.
In this type of vibration motor, it is necessary to apply a voltage of approximately several dozens to several hundreds volts to a piezoelectric vibrator. Accordingly, the applied voltage is increased by using an inductance element, such as a coil, and the increased voltage is used as a driving voltage. Although this method is primarily employed only for a traveling-wave vibration motor, it may also be employed for a standing-wave vibration motor in order to increase the applied voltage. Through intense study by the present inventor, however, it has been proved that a new effect, as well as an increase in the applied voltage, can be exhibited by the above method depending on the selection of an inductance element, such as a coil.
Such an effect will now be explained while referring to an example of conventional standing-wave vibration motors.
Hitherto, as a vibration motor utilizing standing-wave vibration, a vibration motor using standing waves generated in an elastic member is known. In this type of motor, voltages of a specific frequency are applied to a vibration device integrally provided with an elastic member so as to excite a bending vibration and a longitudinal vibration in the elastic member. As a result, a driving force can be obtained via an elliptic-rotation extracting member provided on the elastic member or the vibration device.
A brief explanation will now be given of the principle of driving a standing-wave vibration motor with reference to FIG. 7. In FIG. 7, there are shown an elastic member 71 formed of an elastic member, and piezoelectric vibrators 72a and 72b as electro-mechanical energy conversion element ports for exciting a longitudinal vibration and a bending vibration in the elastic member 71. The directions in which the vibrators 72a and 72b are polarized are indicated by the arrows shown in FIG. 7. There are also shown elliptic-rotation extracting members 73a and 73b integrally provided on the elastic member 71, electrodes 74a, 74b, 74c and 74d for applying specific-frequency voltages to the piezoelectric vibrators 72a and 72b, and a mobile unit 75. An urging spring 76 is also provided to urge the mobile unit 75 against the elliptic-rotation extracting members 73a and 73b at a predetermined force. A bearing 77 serves to reduce a frictional force between the urging spring 76 and the mobile unit 75 generated due to an urging force exerted by the spring 76.
In the standing-wave vibration motor constructed as described above, upon application of specific-frequency voltages which are 90.degree. out of phase with each other (designated by sin and cos, respectively, in FIG. 7) to the electrodes 74a and 74d and the electrodes 74b and 74c, respectively, the piezoelectric vibrators 72a and 72b repeatedly expand and contract at the respective drive frequencies. This excites a longitudinal vibration and a bending vibration in the elastic member 71, and both vibrations are combined into a synthetic vibration, thereby further inducing the elliptic-rotation extracting members 73a and 73b to rotate in an elliptic motion in the same direction. Then, the urging spring 76 urges the mobile unit 75 to contact the rotation-extracting members 73a and 73b, thereby shifting the mobile unit 75 in the respective directions, for example, indicated by the two headed arrow in FIG. 7.
Although in the example shown in FIG. 7, the bending vibration and the longitudinal vibration are respectively determined to be a fourth-order vibration and a first-order vibration, this is not exclusive. Any vibration mode may be employed as long as a driving force can be obtained.
In the standing-wave vibration motor, two vibration modes (the bending vibration and the longitudinal vibration, in this example) are simultaneously caused to resonate at certain drive frequencies, as noted above. Thus, the vibration motor should be shaped so that the resonant frequencies of the two vibration modes substantially coincide with each other.
Although the vibration motor can be shaped with high precision by machining, it is very difficult to substantially match the resonant frequencies of the two vibration modes, as illustrated in FIG. 9, due to anisotropic characteristics inherent in the materials for the motor, variations in the thickness of the adhesive used for joining the elastic member and the piezoelectric vibration devices, and variations in the hardness of the adhesive after it has been cured caused by differences in curing conditions. FIG. 8 illustrates an equivalent circuit of the standing-wave vibration motor shown in FIG. 7. FIG. 9 illustrates the absolute value .vertline.Y.vertline. of the admittance characteristics of the equivalent circuit. In FIG. 9, reference numeral 91 indicates the absolute value .vertline.Y.sub.1 .vertline. of the bending vibration, while 92 designates the absolute value .vertline.Y.sub.2 .vertline. of the longitudinal vibration. The absolute value .vertline.Y.sub.1 .vertline. of the bending vibration is maximized at a frequency f.sub.1, while the absolute value .vertline.Y.sub.2 .vertline. of the longitudinal vibration is maximized at a frequency f.sub.2.
FIG. 10 illustrates the phase characteristics of the equivalent circuit shown in FIG. 8 with respect to currents in relation to the driving voltages. In FIG. 10, 93 represents phase characteristics .theta..sub.1 (f) of the bending vibration, while 94 depicts phase characteristics .theta..sub.2 (f) of the longitudinal vibration. In the standing-wave vibration motor having the above resonance characteristics, a current phase difference d.theta.(f) 95 (.theta..sub.1 (f)-.theta..sub.2 (f)) between the phase .theta..sub.1 (f) 93 of the bending vibration and the phase .theta..sub.2 (f) 94 of the longitudinal vibration with respect to currents in relation to the driving voltages exceeds 180 degrees, as illustrated in FIG. 11, between the two resonant frequencies. It should be noted that FIG. 11 illustrates the actual phase characteristics of a vibration motor obtained by phase-shifting the characteristics of the equivalent circuit shown in FIG. 10 by 90 degrees after considering that there is a 90.degree. phase difference between the bending vibration and the longitudinal vibration in a motor for practical use. As a consequence, there is generated a disparity in the direction in which the vibrator is rotated in an elliptic manner between the interval between the two resonant frequencies and frequency ranges other than the above interval. This disadvantageously narrows the frequency range in which a thrust (velocity) can be obtained to drive the motor, as shown in FIG. 12, and accordingly lowers the vibration efficiency.